A FCBGA (flip-chip ball grid array) semiconductor package is formed with both a flip-chip structure and a ball grid array in which at least one chip is mounted and electrically connected to a surface of a substrate by a plurality of solder bumps, and a plurality of solder balls are implanted on an opposite surface of the substrate to serve as input/output (I/O) connections of the semiconductor package. In order to dissipate heat produced from operation of the chip, a heat sink is usually incorporated in the semiconductor package, as disclosed by U.S. Pat. Nos. 5,311,402, 5,637,920, 5,931,222 and 6,011,304. This heat sink is attached to the substrate by means of an adhesive or solder and is usually greater in surface area than the chip, such that the heat sink covers the chip and effectively dissipates the heat from the chip. Besides the heat sink, at least one passive component can also be mounted on the substrate to enhance electrical performances of the semiconductor package. However, the provision of passive component undesirably decreases area on the substrate available for contact with the heat sink, making the heat sink relatively difficult to be firmly adhered and positioned onto the substrate and thereby leading to dislocation of the heat sink. This situation becomes severe in the use of a large heat sink. Moreover, attachment between the heat sink and the substrate via the adhesive or solder would be damaged by unsatisfactory cleanness of contact surfaces between the heat sink and the substrate, or by undesirable stress applied to the heat sink and the substrate. In this case, delamination may occur at an interface between the heat sink and the substrate and results in dislocation of the heat sink. When the substrate mounted with the heat sink is subject to external force such as vibration or shock, the heat sink may also be dislocated from the substrate.
To solve the above dislocation problem, U.S. Pat. No. 6,093,961 teaches to mount a heat sink on a chip and mechanically secure the heat sink in position. Referring to FIG. 11, the heat sink 50 is directly stacked on and coupled to the chip (flip chip) 52 on a substrate 53. The heat sink 50 is formed with a plurality of flexible legs 51 extending toward the chip 52, each leg 51 having a hook end. When the heat sink 50 is pressed on a top surface 53 of the chip 52, the hook ends of the flexible legs 51 are adapted to be engaged with underside corners of the chip 52, such that the heat sink 50 is securely disposed on the chip 52. However, this structure is defective in that the chip 52 may be damaged if improperly pressing the heat sink 10 on the chip 52. Further, during a high temperature process or thermal cycle, due to mismatch in coefficient of thermal expansion (CTE) between the heat sink 10 and the chip 52, the chip 52 may suffer thermal stress and crack.
Referring to FIG. 12, U.S. Pat. Nos. 5,396,403 and 5,926,371 propose to position a heat sink 60 on a substrate 61 in a mechanical manner that the heat sink 60 is formed with a plurality of holes 62 at positions supposed to be in contact with the substrate 61 that is also formed with a plurality of corresponding holes 63, and a plurality of fixing members 64 such as bolts are used to couple the corresponding pairs of holes 62, 63 and thereby connect the heat sink 60 and the substrate 61. Moreover, referring to FIG. 13, in U.S. Pat. No. 6,441,485, extending portions 71 with hook ends are formed at edges of a heat sink 70, allowing the hook ends to be inserted into corresponding holes 73 formed in a substrate 72, thereby secure the heat sink 70 to the substrate 72.
However, the above mechanical methods of using the fixing members or extending portions for securing the heat sink to the substrate render significant problems. One is that a portion of area on the substrate is predetermined for use to form the holes, which affects circuit layout on the substrate and makes the substrate not able to be mounted with a full array of solder balls. Formation of the holes undesirably increases fabrication costs and process complexity of the substrate. And external moisture or contaminant may enter the holes of the substrate and degrade reliability of fabricated products.
Referring to FIG. 14, in order to improve attachment between a heat sink 80 and a substrate 81, at least one groove 82 is formed on a contact surface of the heat sink 80 for providing more contact area between the heat sink 80 and an adhesive material 83 used to attach the heat sink 80 to the substrate 81. Unfortunately, in practice, the heat sink 80 cannot be effectively securely attached to the substrate 81 only by means of the groove 82.
FIG. 15 shows a semiconductor package with a heat sink 90 having at least one modified groove 91. The groove 91 is shaped as a reverse trapezoid such that an adhesive material 92 received therein is capable of providing an anchoring effect to enhance the attachment between the heat sink 90 and a substrate 93. However, it is technically difficult to form the reverse-trapezoid groove 91 on a contact surface of the heat sink 90.
Therefore, the problem to be solved herein is to enhance adhesion between a heat sink and a substrate to securely position the heat sink on the substrate without damaging a chip mounted on the substrate or affecting circuit layout on the substrate.